GB2312133A

GB2312133A - Determining radio channel quality

Info

Publication number: GB2312133A
Application number: GB9711841A
Authority: GB
Inventors: Yasushi Okada
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1994-10-27
Filing date: 1995-07-07
Publication date: 1997-10-15
Anticipated expiration: 2015-07-07
Also published as: GB9711841D0; GB2312133B

Abstract

When a detector 44 senses a silence interval in voice signals to be transmitted, a generator 48 inserts a pseudo-noise pattern signal into the silence interval for transmission. This may be done at both a mobile station (Fig.1) and at a base station (Fig.2). Each station on detecting the inserted PN signal extracts that signal and relaces it in the forwarded voice signal with a signal corresponding to the background noise of a voice silence state. The extracted PN signal is used for determining channel quality. If the mobile determines that the signal level received from another base station is higher than that from the present base station, it may request a handover but only if the present channel quality is below an acceptable level as evaluated from the BER in the extracted PN signal from the present base station. A handover decision may also involve the present base station determining reception level and evaluating channel quality from the PN signal received from the mobile.

Description

DIGITAL MOBILE TELEPHONE COMMUNICATION APPARATUS The present invention relates to digital mobile telephone communication apparatus in which voice is digitized for radio camunication between a mobile station and a base station The quality of a communication channel between a digital mobile telephone terminal and base station equipment is measured by inserting a pseudo noise (PN) pattern into the communication channel (which carries communication data when in use). however, insertion of a PN pattern into a communication channel is permitted only while the channel is not in use for actual service. When the communication channel is in use, the channel quality can be evaluated only by an error rate detection method using a CRC error check code or a fixed pattern of several bits.

Communication channel switching during communication usually takes place when the mean value of the current reception level drops below a certain value or when the signal level from a base station in an adjacent area becomes greater than the current signal level. A decrease in the received signal level, however, does not necessarily mean a drop in channel communication quality, and in a poor radiowave environment, even when the channel quality is good, unnecessary communication channel switching often occurs, causing unnecessary operations in switching equipment, etc.

In previously-considered systems, for transrriission and reception of character information concurrently with voice signals, two separate channels have to be provided regardless of the transmission rate of the character information. This has often resulted in low channel utilization.

According to a first aspect of the present invention there is provided a mobile unit, for use in a digital mobile telephone communications network, including: transmission means for sending a transmission signal including a digitized voice signal to a base station of the network; voice detector means connected for receiving a voice signal, produced in the mobile unit for transmission to the base station, and operable to detect a silence interval in the received voice signal; and data signal insertion means operable, upon detection of such a silence interval by the voice detector means, to insert a data signal in the transmission signal in place of the said digitized voice signal during the said silence interval, which data signal includes a pseudo-noise pattern signal for use in measuring the quality of the communication channel between the mobile unit and the base station.

According to a second aspect of the present invention there is provided a base station, for use in a digital mobile telephone communications network, including: transmission means for sending a transmission signal including a digitized voice signal to a mobile unit of the network; silence interval detection means connected for receiving a voice signal for transmission to the mobile unit and operable to detect a silence interval in that received voice signal; and data signal insertion means operable, upon detection of such a silence interval by the silence interval detection means, to insert a data signal in the transmission signal in place of the said digitized voice signal during the silence interval, which data signal includes a pseudo-noise pattern signal for use in measuring the quality of the communication channel between the base station and the mobile unit.

By transmitting a data signal including a pattern signal over the voice signal channel during a silence interval of the voice signal, an accurate measurement of communication channel quality is possible during communication, and unnecessary channel switching can be prevented.

Reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a block diagram of a mobile unit for use according to a first embodiment of the present invention; Figure 2 is a block diagram of a base station for use according to the first embodiment of the present invention; Figure 3 is a timing chart illustrating operation when a transition is made from a talking state to a silence state in the first embodiment; Figure 4 is a timing chart illustrating operation when a transition is made from a silence state to a talking state in the first embodiment; Figure 5 is a flowchart illustrating a procedure for communication channel switching in an embodiment of the present invention; and Figure 6 is a diagram illustrating an actual channel switching operation.

The mobile unit shown in Figure 1 inserts a pseudo noise (PN) pattern in a voice signal for transmission on an outgoing channel to a base station, to measure the communication quality of the outgoing channel, and extracts a PN pattern, inserted at the base station in a voice signal for transmission on a channel to the mobile station, to test the incoming channel from the base station.

First, circuit blocks for implementing the basic functions of a mobile unit for digital mobile telephone communication will be described. In Figure 1, the voice signal picked up by a transceiver 10 is amplified by an amplifier 12, and converted by an A/D converter 14 into a digital signal. The digital signal is output from the A/D converter 14 in a repetitive form at a voice sampling rate -(for example, at intervals of 125 zsec), and the timing is changed by a timing converter 16 to generate bursts at intervals of 5 msec, for example, to match the format of time-division multiple access (TDLk). The output of the timing converter 16 is directed through a selector 18 to a TD > k section 20 where prescribed header, etc. are appended to form TDMk bursts. The TDMk bursts output from the TDMA section 20 are passed through a modulator 22, a transmitter 24, a hybrid 26, and are radiated from an antenna 28. Two antennas 28 are provided for space-diversty reception. Signals from the base station, received by the antennas 28, are passed through the hybrid 26 and directed through a receiver 30 and a demodulator 32 and on to a TDM,A section 34. The TDMA section 34 extracts from the received bursts only the bursts that fall within the time slot assigned to the mobile unit, and outputs the extracted bursts with headers removed. The output of the TDMA section 34 is fed to a timing converter 38. The timing converter 38 changes the signal timing to convert the burst digital voice signal into a digital voice signal with discrete intervals by reversing the process performed in the timing converter 16. The output of the timing converter 38 is converted by a D/A converter 40 into an analog signal which is fed to the transceiver 10 via an amplifier 42. A timing generator 43 generates a timing signal necessary for the operation of each circuit block, based on the Output c an oscillator 41 which is synchronized to the timing signal contained in the signals received from the base station.

A voice detector 44 detects z silence interval between voice signals from the output of the amplifier 12, and controls the selector 18 to select, during the silence interval, the outputs of a silence notification frame generator 46 and PN pattern data generator 48 rather than the output of the timing converter 16. The silence notification rame generator 46 outputs a silence notification frame consisting of z start signal andean end signal which are respect~vely appended at the head and tail of PN pattern data output rom the PN pattern data generator 48.

A silence notification or -zze cetector 50 recognizes the start and end of @@ pattern date by detecting 2 start signal and an end signal rom a burst signal output from tne TDMA section 34, and controls a N pattern detector 52 to detect a bit error in the received PN pattern data, and 2 selector 36 to select background noise from 2 background noise insert 54 (i.e. a digitized voice signal corresponding to a silence state)rather than the PN pattern data output from the TDMA section 34. A channel quality monitor 56 computes a bit error rat (BER) by counting the number of error bits detected by the PN pattern detector 52, and notifies a centralized controller 58. The centralized controller 58 comprises a CPU, and makes a decision as to whether communication channel switching be performed, as will be described later, based on the BER value notified from the channel quality monitor 56 and on the value o the received signal level output from the receiver 30 and converted by an A/D converter 60 into 2 dig.tal value.

Figure 2 is 2 block diagram showing the configuration of a base station used in conjunction with the mobile unit of Figure 1 The The same components as those of the mobile unit shown in Figure 1 are designated by like reference numerals, and explanation of such components will not be repeated here.

A network interface 61 is used to interface the base station to a public network, etc. The network interface 61 transfers a digitized voice signal, received from a public network, etc., to the timing converter 16, and also transmits the digitized voice signal from the timing converter 38 cut to the public network, etc. A timing generator 62 generates a timing signal necessary for the operation of each circuit block, based on the output of an oscillator 6t which is synchronized to the timing signal from the public network, etc.

A background noise detector 66 detects a silence interval by detecting the digitized voice signal corrsponding to the background noise or the silence state from the bursts output from the timing converter 16 and, as with the voice detector 44 in Figure 1, controls the selector 18 to select during the silence interval the outputs of the silence notification frame generator 46 and PN pattern data generator 48 rather than the output of the timing generator 16.

A centralized controller 68 notifies the public network, etc., via the network interface 61, of the BER value of the outgoing channel, calculated and supplied by the channel quality monitor 56, and the value of the received signal level of the outgoing line, output from the A/D converter 60.

Figure 3 is a timing chart illustrating the operation when the voice signal changes from a talking state to a silence state at the mobile unit and base station shown in Figures 1 and 2.

As long as the voice signal is present at the transmitting end as shown in Figure 3(A), digitized voice signals are output at intervals of 5 msec from the selector 18 (Figures 1 and 2) as shown in Figure 3(B), and transmitted as TDMA bursts to the receiving end. When the voice signal ceases and enters into a silence state, and the silence state is detected by the voice detector 44 (Figure 1) or the background noise detector (Figure 2), a PN pattern data start signal containing the voice signal being output up to the detection of the silence state is output from the selector 18 in synchronism with the timing of the next burst, as shown in Figure 3(B), and at each subsequent burst timing, PN pattern data is output. When the silence notification frame detector 50 (Figures 1 and 2) at the receiving end has detected the start signal, BER measurement based on bit error detection is initiated by the PN pattern detector 52 (Figures 1, 2) as shown in Figure 3(C). During that period, the background noise output from the background noise inserter 54, i.e. the digital data corresponding to the silence state, is selected by the selector 36 for output.

Figure 4 is a timing chart illustrating the operation when a transition is made from a silence state to a talking state. When no voice is present as shown in Figure 4(A), PN pattern data is transmitted from the transmitting end as shown in Figure 4(B), and BER measurement is performed at the receiving end as shown in Figure 4(C). When the voice detector 44 (Figure 1) or the background noise detector 66 (Figure 2) at the transmitting end detects the start of voice, an end signal containing the voice signal being output until that time is output from the selector 18 in synchronism with the timing of the next burst, and after that, voice signal bursts are output. When the silence notification frame detector 50 (Figures 1 and 2) at the receiving end has detected the end signal, the BER measurement ----------- is stopped, as shown in Figure 4 (C), and the received voice signal bursts are output from the selector 36.

Figure Sis a flowchart illustrating the procedure for communication channel switching that the centralized controller 58 of Figure 1 performs on the basis of the measured value of BER. In step 1000, reception level A from a base station in an adjacent zone, a different base station than the serving base station, is measured using an empty slot, and in step 1002, reception level B of the cuErently-sed base station is measured. If A is greater than B (step 1004), then it is judged whether the channel quality evaluated on the basis of the BER measured at the mobile unit as previously described is within an allowable range in terms of communication quality (step 1006). If the channel quality is at a satisfactory level despite A being greater than B, a request for communication channel switching is not issued and the process returns to step 1000. If the channel quality is outside the allowable range, then a request is made to the network for communication channel switching (step 1008).

An actual channel switching operation will be described with reference to Figure 6. As described in Figure 5, the mobile unit measures and compares the reception levels between the current zone and other zones, and monitors the communication quality of the incoming channel (steps 1100 to 1103). If it is decided that channel switching is necessary, a request for channel switching is sent to the base station (step 1104). The base station then transmits a radio condition request to the mobile station (step 1105) and, based on a radio condition response, obtains information on the current communication channel quality and the current reception level (step 1106). At the same time, the base station evaluates the reception level and communication quality of the outgoing channel (steps 1107, 1108), and makes a decision as to whether channel switching is necessary or not (step 1109).

If it is decided that channel switching is necessary, the base station sends a reception level measurement request (step 1110). Upon reception of the request, the mobile station measures the reception levels from the adjacent zones as well as the current zone over again, and notifies the base station of the reception levels from the current and adjacent zones as a reception level measurement response (step 1111). Based on the reception levels thus notified, the base station determines the channel to switch to and transmits this information to the mobile station (step 1112).

Upon reception of the information, the mobile unit establishes synchronization through the specified communication channel (step 1113), and when syncnronization has been achieved with the base station (step 111 & ), synchronization of the communication channel is established. Further, the communication between the mobile station and the base station is established by transmission and reception of a communication channel open signal (steps 1115, 1116).

In this embodiment, detection of the channel quality and reception levels is performed at both the mobile station and the base station, but whether channel switching is necessary or not may be decided based on the detection at one or the other of the communicating stations.

In a previously-considered system, a request for channel switching was issued upon detection of the reception level from another base station becoming greater than the current reception level. On the other hand, a corrmunication channel switching sequence according to an emDodimexlt of the present invention, as long as the current channel quality remains at a satisfactory level, the request for channel switching is not issued even when the reception level from another base station is greater than the current level. This can serve to prevent unnecessary operations of the switching equipment, etc.

As described above, in embodiments of the present invention, communication channel qualiy can be monitored in real time during communication in a digital mobile communication system. Furthermore, since the request for communication channel switching is issued based on the results of channel quality measurements, such an embodiment can serve to ------ prevent ---- unnecessary operations of the switching equipment, etc., while ensuring reliable channel assignments.

Thus embodiments of the present invention can rrake a substantlcl contribution to inprovements in the reliability of tne digital mobile communication system.

Claims

1. A mobile unit, for use in a digital mobile telephone communications network, including: transmission means for sending a transmission signal including a digitized voice signal to a base station of the network; voice detector means connected for receiving a voice signal, produced in the mobile unit for transmission to the base station, and operable to detect a silence interval in the received voice signal; and data signal insertion means operable, upon detection of such a silence interval by the voice detector means, to insert a data signal in the transmission signal in place of the said digitized voice signal during the said silence interval, which data signal includes a pseudo-noise pattern signal for use in measuring the quality of the communication channel between the mobile unit and the base station.

2. A mobile unit as claimed in claim 1, wherein the said voice signal received by the voice detector means is an analog voice signal.

3. A base station, for use in a digital mobile telephone communications network, including: transmission means for sending a transmission signal including a digitized voice signal to a mobile unit of the network; silence interval detection means connected for receiving a voice signal for transmission to the mobile unit and operable to detect a silence interval in that received voice signal; and data signal insertion means operable, upon detection of such a silence interval by the silence interval detection means, to insert a data signal in the transmission signal in place of the said digitized voice signal during the silence interval, which data signal includes a pseudo-noise pattern signal for use in measuring the quality of the communication channel between the base station and the mobile unit.

4. A base station as claimed in claim 3, wherein the voice signal received by the silence interval detection means is a digitized voice signal and the silence interval detection means are operable to detect when the received digitized voice signal corresponds to a silence interval.

5. A mobile unit, for use in a digital mobile telephone communications network, having means for transmitting, in a silence interval, a pseudo-noise pattern signal for use in measuring the quality of the communication channel between the mobile unit and a base station of the network, substantially as hereinbefore described with reference to the accompanying drawings.

6. A base station, for use in a digital mobile telephone communications network, having means for transmitting, in a silence interval, a pseudo-noise pattern signal for use in measuring the quality of the communication channel between the base station and a mobile unit of the network, substantially as hereinbefore described with reference to the accompanying drawings.